1. Field of the Invention:
This invention relates to an improved apparatus for obtaining core samples of subterranean formations. More specifically, the invention relates to an improved hydraulic motor useful for drilling core cuttings from the sidewall of a drill hole or the like.
2. Description of the Prior Art:
In U.S. Pat. No. 4,354,558, an apparatus and method for drilling into the sidewall of a drill hole is disclosed and claimed. This apparatus was developed and is commercially used to obtain true samples of subterranean formations and for recovering portions of their contained fluids. Formation samples removed from downhole strata by such techniques and apparatus are useful in evaluating the geological, mineralogical and physical characteristics of the formation of interest.
As described in this patent, the sidewall coring tool is contained within a suitable housing which can be lowered, for example, on a standard 7 conductor wireline logging cable through a drill hole to a formation of interest as detected by a gamma ray logging device connected to the housing. The apparatus is then suspended at the formation of interest and by signal from a control panel, a hydraulic means is activated to wedge the apparatus within the drill hole. A hydraulic motor with a core cutting head attached thereto through a core retaining barrel is activated at the control panel for rotating the cutting head. By separate signal from the control panel, a hydraulic means is activated to move the cutting head into cutting engagement with the sidewall of the drill hole. Upon completion of cutting the core, the core retaining barrel and cutting head are deflected to break the core from the formation. the core retaining barrel containing the core is then retracted to within the housing, and the housing is removed from the drill hole. Separate hydraulic systems are provided for operating the hydraulic motor and for operating the hydraulic means. Electricity for operating the hydraulic system is provided through six of the conductors of the wireline logging cable and the seventh conductor is used for control and monitoring. In this manner, highly useful, true specimens of the formation rock including endogenous fluids can be retrieved and subsequently analyzed.
Although the apparatus and method disclosed in U.S. Pat. No. 4,354,558 has proved to be commercially successful, problems still exist with respect to providing and maintaining a reliable and powerful compact hydraulic motor in such a confined hostile environment associated with downhole oil and gas well applications. The present invention is viewed as being a specific improvement in the hydraulic motor of the sidewall coring tool described in U.S. Pat. No. 4,354,558.